There is a three-axle truck that is used, for example, in a locomotive or the like, and has three wheelsets in the one truck. In this three-axle truck as well as in a two-axle truck, axle boxes of each of the wheelsets are supported through axle springs by side frames of a truck frame so as to be vertically movable, so that a carbody load of a locomotive or the like is distributed to rails through the truck frame, each of the axle springs and each of the wheelsets. Here, all the axle springs used on the respective wheelsets are the same. Respective axle loads on these three wheelsets need to be the same for the following reason.
In the locomotive, the respective wheelsets are basically driven with the same tractive torque, and thus, if the respective axle loads on the three wheelsets are uneven, wheel slip is caused in the wheelset having the smaller axle load because of a lower adhesion force to a rail. This wheel slip may cause a degradation of the tractive effort of the locomotive and a damage of a wheel tread and a rail surface.
Moreover, even with a trailing truck not equipped with an electric motor, there are also similar problems in a freight car with braking force control in which a braking force is adjusted in response to a weight applied to the truck of the freight car. That is, if the respective axle loads on the three wheelsets are uneven, sliding is caused at the time of braking control in the wheelset having the smaller axle load because of a lower adhesion of the wheelset in the smaller axle load to the rail. As a result, the damage of the wheel tread and the rail surface are caused.
The unevenness of the axle loads, which causes the above-described problem, is caused in a three-axle truck by a truck frame deflection with a concave shape caused by a carbody load F acting on a central portion of a truck frame 10 as shown in FIG. 5. As a result of this deflection, an axle load r2 on a middle wheelset 2 of the three wheelsets becomes larger than an axle load r1 on each of the wheelsets 1, 3 at both ends.
As one of solutions of the above-described problem, there is a technique of inserting a liner between each axle box and each axle spring in each of the wheelsets located at both the ends of the three-axle truck to equalize the imposed loads on the respective axle springs of the three wheelsets.
Moreover, there also exists a technique of providing equalizer beams between the axle box of the middle wheelset and the axle boxes of the wheelsets at both the ends, respectively.